1. Field of the Invention
The present invention relates to semiconductor devices and SOI substrates, and particularly to a semiconductor device and an SOI substrate having improved insulating film and improved buried insulating film forming semiconductor elements.
2. Description of the Background Art
With miniaturization of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), attempts are being made to reduce the film thickness of gate insulating films for the purposes of improving the current driving capability and alleviating the roll-off of the threshold voltage (the amount of variation of the threshold voltage caused as the gate length and gate width vary).
There are two reasons behind this:
(1) Improving the current driving capability speeds up operation of the circuitry, thus increasing the operating frequency of the semiconductor chip.
(2) Alleviating the threshold voltage roll-off reduces variations of transistors"" threshold voltages caused as the gate length and gate width vary during the process of transfer etc., thus facilitating the mass production.
Thinning a gate insulating film made of silicon oxide (SiO2) to a thickness of 3 nm or less causes serious gate leakage current which is due to direct tunneling from the silicon substrate to the gate electrode. Therefore the film thickness of about 3 nm is the limit of the silicon oxide gate insulating film. However, there are demands for gate insulating films having film thicknesses of 3 nm or less, calculated in terms of silicon oxide film (referred to as reduced film thickness thereinafter), in order to improve the current driving capability.
Further, when a silicon oxide gate insulating film is formed in contact with a polysilicon film which heavily contains boron (which is used as a gate electrode in a surface-channel P-type MOSFET), boron in the polysilicon film thermally diffuses into the gate insulating film during thermal processing and reaches the channel to cause the threshold voltage to vary.
As a method for solving this problem, a MOSFET 90 as shown in FIG. 43 is used in the generation of gate length of 0.12 xcexcm or less, for example.
In FIG. 43, the MOSFET 90 has a gate insulating film composed of a two-layer film of a silicon oxide film 11 and a silicon nitride film 12 formed in order on the silicon substrate 1, and a gate electrode composed of a three-layer film of a doped polysilicon film 13, a barrier metal layer 14 (WNx, TiNx, Ta, TaN, etc.) and a metal film 15 formed in order on the silicon nitride film 12. The gate insulating film formed of a silicon oxide film and a silicon nitride film is referred to as ON (Oxide-Nitride) film hereinafter.
The MOSFET 90 has a coating insulating film 16 covering the gate insulating film and gate electrode, a sidewall insulating film 17 covering at least the sides of the coating insulating film 16, a channel layer 7 provided in the surface of the silicon substrate 1 under the gate electrode, a pair of extension layers 6 facing each other through the channel layer 7, pocket layers 5 provided in the pair of extension layers 6, and a pair of main source/drain layers 4 adjacent to the pair of extension layers 6. Although the extension layers 6 should be referred to as source/drain extension layers 6 since they have the same conductivity type as the main source/drain layers 4 and function as source/drain layers, they are called extension layers 6 for convenience.
The active region of the MOSFET 90 is defined by an STI (Shallow Trench Isolation) film 3, a kind of element isolation insulating film. A channel stopper layer 2 is provided in the silicon substrate 1 and a first interlayer insulating film 21, an insulating film 22, a second interlayer insulating film 23 and a third interlayer insulating film 24 are deposited over the MOSFET 90.
FIG. 43 shows a structure including contacts 31 passing through the first interlayer insulating film 21 and the insulating film 22 to reach the pair of main source/drain layers 4, a first interconnection layer 32 connected to one of the contacts 31, a contact 33 passing through the second interlayer insulating film 23 to reach the other contact 31, and a second interconnection layer 34 connected to the contact 33. This structure is just an example and other structures are also possible.
FIG. 44 shows, for reference, dopants used in individual layers of MOSFETs. FIG. 44 classifies N-type MOSFET and P-type MOSFET each into surface channel type and buried channel type and lists dopants which can be used in the channel layer, channel stopper layer, main source/drain layers, extension layers, pocket layers and doped polysilicon layer.
Next, advantages of the above-described ON film are described. The ON film has the following two advantages:
(1) Under the condition that the gate current due to direct tunneling hardly flows, the reduced film thickness can be made thinner than 3 nm.
(2) It is free from variation of the threshold voltage caused by thermal diffusion of dopant in polysilicon: the dopant in polysilicon does not thermally diffuse in the gate insulating film to reach the channel since the dopant diffusion coefficient in the silicon nitride is much smaller than that in silicon oxide.
While attempts have been made to form a silicon nitride film as the gate insulating film on the silicon substrate, this strategy has not been put in practice since the interface state density increases at the silicon nitride/silicon substrate interface. When the interface state density increases, the mobility and effective carrier density decrease as carriers moving in MOSFET repeat trap/de-trap, which reduces the drain current. This in turn reduces the operating speed of the semiconductor integrated circuit formed of the MOSFETs.
While the ON film has many advantages as described above, it has some problem with hot carrier resistance.
FIGS. 45 to 47 are schematic diagrams used to explain the mechanism of hot-carrier-induced deterioration of an ON film formed on a silicon substrate. Hydrogen atoms are introduced into the ON film during formation of the silicon oxide film or during subsequent processing (hydrogen sintering etc.) and they combine with part of silicon atoms in the silicon oxide film of the ON film as shown in FIG. 45. FIG. 45 shows bonded structures of a silicon atom (Si) and a hydroxyl group (OH). Three atoms shown by R are bonded to a silicon atom by single bond. This shows that three atoms of oxygen (O), hydrogen (H), silicon, etc. are bonded through single bond. This expression is used also in FIGS. 47 and 48.
Hydrogen atoms are introduced also into the silicon nitride film during formation of the film or subsequent processing. The hydrogen atoms introduced in the process of hydrogen sintering etc. join and terminate dangling bonds of silicon atoms at the SiO2/Si interface.
When stress voltage is applied to the MOSFET (for example, with an N-type MOSFET, power-supply voltage VDD to the drain and gate and 0 V or base power-supply voltage VBB=xe2x88x921 V to the source), hot carriers HOT in the silicon substrate, which have been accelerated by the internal electric field and gained energy larger than the barrier energy at the SiO2/Si interface, pass through the interface into SiO2 as shown in FIG. 45.
Due to the energy of the hot carriers HOT, the bonds of hydrogen atoms of hydroxyl groups bonded to silicon atoms are cut and the dangling bonds of oxygen function as fixed charges.
As shown in FIG. 46, the hydrogen atoms freed from the bonds reach the SiO2/Si interface because of drift caused by the electric field in the gate insulating film or thermal diffusion. The hydrogen atoms which have arrived at the interface react with the combined structure of Si atoms and hydrogen atoms at the interface to form hydrogen molecules.
These hydrogen molecules volatilize as gas and as shown in FIG. 47 the dangling bonds of silicon atoms at the SiO2/Si interface function as interface states, and the dangling bonds of silicon atoms in the silicon oxide film function as fixed charges.
Formation of the fixed charges or interface states causes threshold voltage variation, drain current deterioration, etc., which reduces the operating speed of the circuit or causes malfunction of the circuit.
While hydrogen atoms in the silicon oxide film deteriorate the ON film through the mechanism described above, hydrogen atoms in the silicon nitride film deteriorate the ON film through the following mechanism.
The silicon nitride film of the ON film is usually formed by chemical reactions expressed by Formulas (1) and (2).
3SiH2Cl2(g)+4NH3(g)xe2x86x923Si3N4(s)+6HCl(g)+6H2(g)xe2x80x83xe2x80x83(1)
3SiH4(g)+4N*(g)xe2x86x92Si3N4(s)+6H2(g)xe2x80x83xe2x80x83(2)
Formula (1) shows a reaction in a CVD reaction device or RTN (Rapid Thermal Nitridation) device and Formula (2) shows a reaction by plasma excitation. N* in Formula (2) represents the radical of a nitrogen atom.
As can be seen from Formulas (1) and (2), hydrogen gas is produced as a byproduct during formation of the silicon nitride film. Although it is represented as hydrogen molecules in the formulas above, part of them are introduced into the silicon nitride film during the reaction in the form of hydrogen atoms. Hydrogen atoms in the silicon nitride film exist in various forms: some may be combined with silicon atoms and some may exist in interstices of the silicon nitride lattice, for example.
FIG. 48 is a diagram showing the dependence of hydrogen atom content in silicon nitride film formed by using the reaction shown by Formula (1) on the partial pressure of ammonia gas; the horizontal axis shows the ratio of the partial pressure of ammonia gas to the total pressure in the reaction chamber and the vertical axis shows the hydrogen atom content (atomic %).
As can be seen from FIG. 48, the silicon nitride film contains hydrogen atoms of about 10 to 30 atomic %.
When an ON film is used as a gate insulating film, hydrogen atoms in silicon nitride move into the silicon oxide film because of drift or diffuse when a stress voltage is applied; as shown in FIG. 46, as well as hydrogen atoms in the silicon oxide film, they react with hydrogen atoms in hydroxyl groups bonded to silicon atoms to form hydrogen molecules, or react with the combined structure of Si atoms and hydrogen atoms at the SiO2/Si interface to form hydrogen molecules.
The hydrogen molecules volatilize as gas and, as shown in FIG. 47, the dangling bonds of silicon atoms at the SiO2/Si interface function as interface states and the dangling bonds of oxygen atoms in the silicon oxide film function as fixed charges. The gate insulating film composed of ON film thus has a property that it is deteriorated faster than a gate insulating film composed only of a silicon oxide film.
Particularly, the current tendency is toward thinner silicon oxide film and thicker silicon nitride film in order to lessen the reduced film thickness of ON film, so that the deterioration caused by hydrogen atoms in the silicon nitride film has become dominant to cause problems which cannot be neglected.
Further, semiconductor devices are being increasingly systematized and semiconductor devices having various functional blocks are now available, where different maximum voltages are applied to individual functional blocks such as a memory array section, input/output section, CPU section and logic section. It is therefore becoming difficult to satisfy the reliability of all gate insulating films with the same ON film.
The hot carrier resistance problem arises not only in such a gate insulating film as described above but also in an element isolation insulating film for making element isolation with a trench isolation structure such as STI film and in a buried oxide film in an SOI substrate (Silicon On Insulator), for example.
A first aspect of the present invention is directed to a semiconductor device having at least one kind of MOSFET, wherein the MOSFET comprises a gate insulating film provided on a main surface of a semiconductor substrate and a gate electrode provided on the gate insulating film, and the gate insulating film comprises a first two-layer film including a silicon oxide film and a silicon oxynitride film, at least one of the silicon oxide film and the silicon oxynitride film containing deuterium atoms, or a second two-layer film including a silicon nitride film and a silicon oxynitride film, at least one of the silicon nitride film and the silicon oxynitride film containing deuterium atoms.
Preferably, according to a second aspect, in the semiconductor device, the silicon oxynitride film in the first two-layer film is formed on the silicon oxide film, and the silicon nitride film in the second two-layer film is formed on the silicon oxynitride film.
Preferably, according to a third aspect, in the semiconductor device, the silicon oxynitride film in the first two-layer film is thicker than the silicon oxide film.
Preferably, according to a fourth aspect, in the semiconductor device, the silicon oxynitride film in the second two-layer film is thicker than the silicon nitride film.
Preferably, according to a fifth aspect, in the semiconductor device, in the first and second two-layer films, their respective first layers and second layers contain deuterium atoms.
Preferably, according to a sixth aspect, the semiconductor device has a plurality of functional blocks to which different maximum applied voltages are applied, and the at least one kind of MOSFET comprises a plurality of kinds of MOSFETs having the gate insulating films differing in thickness, wherein the plurality of kinds of MOSFETs are allotted to the plurality of functional blocks in accordance with the thicknesses of their gate insulating films so that the MOSFETs can keep the enough reliability subject to the respective maximum applied voltages.
Preferably, according to a seventh aspect, in the semiconductor device, the at least one kind of MOSFET further comprises a coating insulating film covering the multi-layered structure of the gate insulating film and the gate electrode and partially covering the main surface of the semiconductor substrate extend outwardly from sides of the multi-layered structure, and a sidewall insulating film covering the coating insulating film, wherein the coating insulating film contains deuterium atoms.
Preferably, according to an eighth aspect, in the semiconductor device, the coating insulating film is a silicon oxide film.
Preferably, according to a ninth aspect, in the semiconductor device, the coating insulating film is a silicon oxynitride film.
A tenth aspect is directed to a semiconductor device having a MOSFET, wherein the MOSFET comprises a gate insulating film provided on an active region defined by an element isolation insulating film provided in a main surface of a semiconductor substrate, and a gate electrode provided on the gate insulating film, wherein the element isolation insulating film comprises a trench formed in the main surface of the semiconductor substrate, an inner-wall insulating film provided on an inner-wall of the trench and containing deuterium atoms, and an insulating film buried in the trench covered by the inner-wall insulating film.
Preferably, according to an eleventh aspect, in the semiconductor device, the inner-wall insulating film is a silicon oxide film which contains deuterium atoms or a silicon oxynitride film which contains deuterium atoms.
Preferably, according to a twelfth aspect, in the semiconductor device, the insulating film is a silicon oxide film which contains deuterium atoms or a silicon oxynitride film which contains deuterium atoms.
Preferably, according to a thirteenth aspect, in the semiconductor device, the inner-wall insulating film has its top edge raised to form a gentle curve on the main surface of the semiconductor substrate and the gate electrode of the MOSFET has its edge in the gate width direction engaged with the top edge of the inner-wall insulating film.
Preferably, according to a fourteenth aspect, in the semiconductor device, the semiconductor substrate is an SOI substrate which comprises a buried insulating film provided on a silicon substrate and an SOI layer provided on the buried insulating film, and the buried insulating film contains deuterium atoms.
According to a fifteenth aspect, an SOI substrate comprises: a buried insulating film provided on a silicon substrate, and an SOI layer provided on the buried insulating film, wherein the buried insulating film is a two-layer film comprising any two of a silicon oxide film, a silicon oxynitride film and a silicon nitride film.
Preferably, according to a sixteenth aspect, in the SOI substrate, the buried insulating film contains deuterium atoms.
Preferably, according to a seventeenth aspect, in the SOI substrate, the buried insulating film comprises a first layer adjacent to the SOI layer and a second layer underlying the first layer, and at least the first layer contains deuterium atoms.
Preferably, according to an eighteenth aspect, in the SOI substrate, the first layer is the silicon oxide film or the silicon oxynitride film.
According to a nineteenth aspect, a semiconductor device at least comprises a MOSFET provided on the SOI layer of the SOI substrate according to the fifteenth aspect.
According to the semiconductor device of the first aspect of the present invention, the gate insulating film has a first two-layer film including a silicon oxide film and a silicon oxynitride film in which at least one of the layers contains deuterium atoms, or a second two-layer film including a silicon nitride film and a silicon oxynitride film in which at least one of the layers contains deuterium atoms. Deuterium atoms, which are heavier than hydrogen atoms, drift or diffuse more slowly than hydrogen atoms from the first layer to the second layer or in the opposite direction. Therefore interface state generation is slow even when a stress voltage is applied. This enhances the reliability of the MOSFET. Further, since the bond energy between deuterium atoms and silicon atoms is larger than that between hydrogen atoms and silicon atoms, the deuterium atoms are less susceptible to dissociation from silicon atoms caused by hot carriers coming from the semiconductor substrate. Hence, forming the first two-layer film or second two-layer film which contain deuterium suppresses the hot-carrier-induced dissociation occurring when a stress voltage is applied, thus improving the hot carrier resistance under stress voltage, which lengthens the life of the MOSFET and improves the reliability.
According to the semiconductor device of the second aspect, the silicon oxide film is formed on the semiconductor substrate when the first two-layer film is used, and the silicon oxynitride film is formed on the semiconductor substrate when the second two-layer film is used. This prevents an increase in interface state density at the interface with the semiconductor substrate.
According to the semiconductor device of the third aspect, the silicon oxynitride film having a larger relative dielectric constant is thicker than the silicon oxide film. This increases the capacitance of the gate insulating film, which in turn improves the operating speed of the circuit.
According to the semiconductor device of the fourth aspect, the silicon oxynitride film is thicker than the silicon nitride film, so that the stress at the substrate interface can be reduced, leading to the reduction of the interface state density and defect density.
According to the semiconductor device of the fifth aspect, in the first and second two-layer films, their respective first layers and second layers contain deuterium atoms. This slows down interface state generation even when a stress voltage is applied, thus improving the reliability of the MOSFET. Further, deuterium atoms are less susceptible to dissociation from silicon atoms caused by hot carriers from the semiconductor substrate. This suppresses the hot-carrier-induced dissociation under stress voltage, thus improving the hot carrier resistance under stress voltage, which lengthens the life of the MOSFET and improves the reliability.
According to the semiconductor device of the sixth aspect, a plurality of kinds of MOSFETs are allotted to a plurality of functional blocks in accordance with the thicknesses of their gate insulating films so that they can keep the reliability subject to the respective maximum applied voltages. The gate insulating films can be adjusted in thickness in accordance with the maximum applied voltages to the plurality of functional blocks by adjusting the film thickness of one layer or both layers in each gate insulating film, so as to optimize the operating speed and reliability for each individual functional block.
According to the seventh aspect, the semiconductor device further comprises a coating insulating film covering the multi-layered structure of the gate insulating film and the gate electrode and partially covering the main surface of the semiconductor substrate extend outwardly from sides of the multi-layered structure, and a sidewall insulating film covering the coating insulating film, and the coating insulating film contains deuterium atoms. The deuterium atoms in the film join and terminate dangling bonds of silicon atoms in the film and join and terminate dangling bonds of silicon atoms at the interface with the silicon substrate, thus reducing the trap density and interface state density. Since the coating insulating film is adjacent to the gate insulating film in some area, the use of the deuterium-containing insulating film which can reduce dangling bonds avoids adverse effects on the gate insulating film.
According to the semiconductor device of the eighth aspect, the coating insulating film is a silicon oxide film. Therefore it can be formed by various methods, as TEOS oxide film, HDP oxide film, thermal oxide film, etc.
According to the semiconductor device of the ninth aspect, the coating insulating film is a silicon oxynitride film which has resistance to oxidation. This prevents the film thickness from varying because of oxidation.
According to the semiconductor device of the tenth aspect, the element isolation insulating film comprises an inner-wall insulating film provided on the inner-wall of a trench and containing deuterium atoms and an insulating film buried in the trench covered by the inner-wall insulating film. Deuterium terminates dangling bonds in the inner-wall insulating film. Dissociation of deuterium atoms from silicon atoms due to hot carriers coming from the semiconductor substrate is less likely to occur, which fact suppresses interface state generation and trap generation at the interface between the inner-wall insulating film and the substrate. Therefore, when a gate electrode is engaged with it, the hot carrier resistance and reliability are improved.
According to the semiconductor device of the eleventh aspect, the inner-wall insulating film is a silicon oxide film which contains deuterium atoms or a silicon oxynitride film which contains deuterium atoms. These films can be formed relatively easily.
According to the semiconductor device of the twelfth aspect, the insulating film is a silicon oxide film which contains deuterium atoms or a silicon oxynitride film which contains deuterium atoms. This provides the effect of preventing deuterium in the inner-wall insulating film from volatilizing in subsequent thermal processing.
According to the semiconductor device of the thirteenth aspect, a top edge of the inner-wall insulating film is raised to form a gentle curve on the main surface of the semiconductor substrate and an edge in the gate width direction of the gate electrode of the MOSFET is engaged with the top edge of the inner-wall insulating film. This prevents the problem that the electric field is concentrated at the edge in the gate width direction of the gate electrode to cause the MOSFET to turn on at voltage lower than the designed value of the threshold voltage.
According to the semiconductor device of the fourteenth aspect, the semiconductor substrate is composed of an SOI substrate and the buried insulating film contains deuterium atoms. The bond energy between silicon atoms and deuterium in the buried insulating film is larger than the bond energy between silicon atoms and hydrogen atoms, which fact suppresses interface state generation and fixed state generation. Thus the reliability of the MOSFET formed on the SOI substrate can be enhanced.
According to the SOI substrate of the fifteenth aspect, the semiconductor substrate is composed of an SOI substrate and the buried insulating film is a two-layer film comprising any two of a silicon oxide film, a silicon oxynitride film and a silicon nitride film. Accordingly, thermal stress can be alleviated by combining a silicon oxide film which generates expansile stress when heated and a silicon nitride film which generates compressive stress, for example. This structure introduces a smaller amount of thermal stress into the SOI layer than a buried insulating film having the same thickness and formed only of a silicon oxide film, which reduces interface state generation at the interface with the adjacent SOI layer. This reduces defects formed during manufacturing processing and reduces the leakage current of the semiconductor device.
According to the SOI substrate of the sixteenth aspect, the buried insulating film contains deuterium. When silicon atoms and deuterium are combined in the buried insulating film, their bond energy is larger than that between silicon atoms and hydrogen atoms, so that interface state generation and fixed state generation are less likely to occur. This improves the reliability of the semiconductor device formed on the SOI substrate.
According to the SOI substrate of the seventeenth aspect, the buried insulating film is divided into a first layer adjacent to the SOI layer and a second layer under the first layer, and at least the first layer contains deuterium atoms. This certainly reduces interface state and fixed state generation at the interface between the SOI layer and the buried insulating film, which improves the reliability of semiconductor device formed on the SOI substrate.
According to the SOI substrate of the eighteenth aspect, the first layer is the silicon oxide film or the silicon oxynitride film. The interface state density can be reduced than in a structure using a silicon nitride film.
According to the semiconductor device of the nineteenth aspect, the thermal stress exerted on the SOI layer can be reduced, which suppresses interface state generation at the interface with the adjacent SOI layer. This suppresses formation of defects during manufacturing processing and hence the leakage current of the MOSFET, thus providing a semiconductor device less susceptible to deterioration of operating characteristics.
The present invention has been made to solve the above-described problems and a first object of the invention is to provide a systematized semiconductor device having a gate insulating film which can be formed thinner than a silicon oxide film and which is less susceptible to deterioration.
A second object of the invention is to provide semiconductor devices with improved reliability in which such insulating films have improved hot carrier resistance.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.